Following is an overview of object-oriented techniques that, in some cases, is specific to the C++ programming language. Two common characteristics of object-oriented programming languages are support for data encapsulation and data type inheritance. Data encapsulation refers to associating functions with data. Inheritance refers to the ability to declare a data type in terms of other data types. Thus, inheritance provides for a hierarchy of classes where the classes that are defined lower in the hierarchy share the structure or behavior of the classes that are higher in the hierarchy. In the C++ programming language, data encapsulation and inheritance are supported through the use of classes. A class is a defined type. A class declaration describes the data members and function members of the class. A function member is also referred to as a method of a class. The data members and function members of a class are bound together in that the function members operate on a particular instance of the data members of the class. An instance of a class is also called an object of the class. Thus, a class provides a definition for a group of objects with similar properties and common behavior.
To allocate storage for an object of a particular type (class), an object is instantiated. The term "instantiating" refers to providing code for member functions and data to data members. Once instantiated, the function members of the particular object can be invoked to access and manipulate the data members. Thus, in this manner, the function members implement the behavior of the object, and the object provides a structure for encapsulating data and behavior into a single entity.
To support the concept of inheritance, classes may be derived from (based upon the declaration of) other classes. A derived class is a class that inherits the characteristics--data members and function members--of its base classes. A class that inherits the characteristics of another class is a derived class. A class that does not inherit the characteristics of another class is a primary (root) class. A class whose characteristics are inherited by another class is a base class. A derived class may inherit the characteristics of several classes; that is, a derived class may have several base classes. This is referred to as multiple inheritance.
A class may also specify whether its function members are virtual. Declaring that a function member is virtual allows the function to be overridden by a function of the same name and type in a derived class. If a virtual function is declared without providing an implementation, then it is referred to as a pure virtual function. A pure virtual function is a virtual function declared with the pure specifier, "=0". If a class specifies a pure virtual function, then any derived class needs to specify an implementation for that function member before an object of that class can be instantiated. A class which contains at least one pure virtual function member is an abstract class.
FIG. 1 is a block diagram illustrating typical data structures used to represent an object. An object is composed of instance data (data members) and function members, which implement the behavior of the object. The data structures used to represent an object comprise instance data structure 101, virtual function table 102, and the function members 103, 104, 105. The instance data structure 101 contains a pointer to the virtual function table 102 and contains data members. The virtual function table 102 contains an entry for each virtual function member defined for the object. Each entry contains a reference to the code that implements the corresponding function member. In the following, an object will be described as an instance of a class as defined by the C++ programming language. One skilled in the art would appreciate that other object models can be defined using other programming languages.
An advantage of using object-oriented techniques is that these techniques can be used to facilitate the sharing of objects. For example, a program implementing the function members of an instantiated object (a "server program") can share the object with another program (a "client program"). To allow an object of an arbitrary class to be shared with a client program, interfaces are defined through which an object can be accessed without the need for the client program to have access to the class definitions at compile time. An interface is a named set of logically related function members ("methods") and data members ("properties"). In C++, an interface is implemented as an abstract class with no data members and whose virtual functions are all pure. Thus, an interface provides a published protocol for two programs to communicate. Interfaces are typically used for derivation: a program defines (implements) classes that provide implementations for the interfaces the classes are derived from. Thereafter, objects are created as instances of these derived classes. Objects instantiated from a derived class implementing particular interfaces are said to "support" the interfaces. An object supports one or more interfaces depending upon the desired functionality.
When a client program desires to share an object, the client program needs access to the code that implements the interfaces for the object (the derived class code). To access the derived class code (also referred to as class code), each class implementation is given a unique class identifier (a "CLSID"). A persistent registry in each computer system is maintained that maps each CLSID to the code that implements the class. Typically, when a spreadsheet program is installed on a computer system, the persistent registry is updated to reflect the availability of that class of spreadsheet objects. So long as a spreadsheet developer implements each function member defined by the interfaces to be supported by spreadsheet objects and so long as the persistent registry is maintained, the client program can access the function members of shared spreadsheet objects without regard to which server program has implemented them or how they have been implemented.
Since an object may support some interfaces and not others, a client program would need to determine at runtime whether a particular object supports a particular interface. To enable this determination, every object supports the interface IUnknown, which contains a function member, QueryInterface, that indicates which interfaces are implemented for the object. The method QueryInterface is defined as follows: EQU virtual HRESULT QueryInterface (REFIID iid, void**ppv)=0;
The method QueryInterface is passed an interface identifier in parameter iid (of type REFIID) and returns in parameter ppv a pointer to the implementation of the identified interface of the object for which the method is invoked. If the object does not support the interface, then the method returns a predefined status value of E.sub.-- NOINTERFACE. The type HRESULT indicates a predefined status.
FIG. 2 is a symbolic representation of an object. In the following description, an object data structure is represented by the shape 201 labeled with the interfaces through which the object may be accessed. These interfaces include the IUnknown interface, the IDatabase interface, and the IBasic interface.
One type of object is known as a control. A "control" is a screen oriented object that displays information to a user and accepts input from the user. Thus, the definition of a control encompasses an edit box, as well as a pushbutton on a computer display. Controls are typically stored in a container. A "container" is responsible for storing controls, as well as presenting the controls to a user via the computer display. Thus, a container is usually an application program such as the Microsoft Word word processing program, the Excel spreadsheet program, or the Visual Basic Development Environment, which are all available from Microsoft Corporation of Redmond, Wash.